The present invention relates to ion implantation of battery components such as electrodes for use in batteries with highly corrosive chemical electrolytes. Among other advantages, this inventive treatment process gives battery components a greatly increased resistance to chemical corrosion during battery operation and storage.
Ion Implantation Techniques
Various forms of ion implantation have been used to improve the properties of surfaces. These modifications can be accomplished using ion implantation to alter the chemical structure of the surface and to mechanically modify the physical structure of the surface. Typically, chemical modification takes the form of insertion of ions of solid material into the surface of a solid body. This technology has been limited to small scale treatment of flat surfaces, such as in semiconductor technology. Mechanical modification is accomplished by producing defects and other physical changes on the surface of a material. This technology allows larger scale applications. The treated substrate is generally a hard metal surface.
The use of ion implantation technology allows the production of mixed material surface layers on bulk metal bodies. These implanted metallic surface layers enjoy considerably better adhesion to the substrate surface than those produced by plating or other techniques because of the gradual change in composition with distance from the surface.
In contrast to alloy methods, the ion implantation of materials has the advantage of conserving expensive materials by limiting their application to the surface of a component rather than being distributed throughout the bulk material.
Ion implantation processing can physically improve the mechanical performance of the object's surface, such as its wear characteristics. Rare earth ions implanted in iron and steel and tin ions implanted in titanium are effective in improving wear resistance. Titanium and carbon ions implanted in steel can reduce the surface friction by a factor of two.
Ion implantation techniques can provide for a surface with specific chemical characteristics. The implantation of the correctly selected metallic atom species can influence the chemical properties of metal surfaces, such as corrosion, oxidation, and catalytic properties.
Ion implantation used for the chemical modification of surfaces has a broad range of applications. For instance, one can produce catalytic surfaces on a solid, non-catalytic bulk material. Using ion implantation, the properties of electronic materials can be controlled in order to manufacture integrated circuits. Ion implanted surfaces have a number of advantages over bulk alloys. The layers are very thin, and the vast reduction in diffusion times means reactions occur faster and at lower temperatures than in conventional metallurgy.
Ion treatment of surfaces during which little or minimal ion implantation occurs has also been demonstrated to produce physical modification of surfaces, in some cases with improved tribiological properties. Surface defects and other modifications can be produced using ion implantation by nobel gasses. In this processing technique the chemical makeup of the treated surface remains substantially unaltered.
The surfaces of certain metals have been treated by ion implantation to improve their physical toughness and corrosion properties under various conditions of hard physical wear. Various research groups have used ion implantation to accomplish a physical toughening of the surfaces of iron (Ashworth, et al, Corrosion Science, 17, 1977, p. 947), aluminum (Natishan, et al, Nuclear Instruments and Methods in Physics Research, B59/60, 1991, p. 841), and steel (Nielsen, et al, Ibid, Vol. 59/69, p. 772).
Both mixed material and physical modification techniques were used by the above cited researchers to accomplished the desired results. A typical example of these research efforts is ion implantation of hard metals with tantalum, which because of its known effect in alloys increases the physical toughness of the target surface. Implantation of a metal's surface with argon, by contrast, appears to increase surface toughness by introducing physical changes to the surface of the material.
Limitations of Batteries With Highly Corrosive Electrolytes
Corrosion of the electrodes and other internal battery surfaces in lead-acid and other highly corrosive electrolyte batteries is a chronic difficulty, and an impediment to broad application of these batteries. Corrosion is a particular problem on the positive plates, and is an important factor that limits the cycle life of a highly corrosive electrolyte battery.
The difficulties in providing for a commercially practical lead-acid battery in view of the corrosive nature of the inner battery environment have been described in various publications. Examples of such reports are from Caniel et al (Electrochim. ACTA, Vol. 20, p. 781, 1983) and Guo et al (Journal of the Electrochemical Society, Vol. 138, p. 1222, 1991).
Examples of other highly corrosive electrolyte batteries with currently limited applicability are molten salt (Li/FeS.sub.2), and sodium sulfur (Na/S) batteries. The electrodes of these batteries are typically of a complex three-dimensional structure with metal mesh current collectors and metal cases. In the case of lead acid batteries, the electrode is composed of a soft, malleable material. All of these electrodes are subject to substantial corrosion due to both their high electrical potential and the highly corrosive nature of their electrolyte. Other battery components, such as metal battery cases, also need protection.
It would be an important advancement in the lead-acid and highly corrosive electrolyte battery art if it were possible to substantially decrease the corrosion rate of metallic surfaces in such batteries.